Testing RX operations with delay

CountdownTimer.kt

fun countdownTimer(
    duration: Long,
    scheduler: Scheduler = Schedulers.io()
) = Observable.interval(1, TimeUnit.SECONDS, scheduler)
    .takeWhile { it < duration }
    .map { duration - it }

CountdownTimerTest.kt

class CountdownTimerTest {
    @Test
    fun test() {
        val testScheduler = TestScheduler()
        val duration = 10L

        val testObserver = countdownTimer(duration, testScheduler).test()

        testScheduler.advanceTimeBy(duration + 1, TimeUnit.SECONDS)
        testObserver
            .assertValues(10, 9, 8, 7, 6, 5, 4, 3, 2, 1)
            .assertComplete()
    }
}

Main.kt

fun main() {
    val duration = 10L
    val disposable = countdownTimer(duration)
        .subscribe(
            {
                println("$it")
            },
            {
                println("[ERROR] $it")
            }
        )

    Thread.sleep(TimeUnit.SECONDS.toMillis(duration + 1))
    disposable.dispose()
}